Modern high-efficiency combustion turbines have firing temperatures that exceed about 2300° F. (1093° C.), and firing temperatures continue to increase as demand for more efficient engines continues. Many components that form the combustor and turbine (or “hot gas path”) sections are directly exposed to aggressive hot combustion gases, for example, the combustion liner, the transition duct between the combustion and turbine sections, and the turbine stationary nozzles and rotating buckets and surrounding ring segments. In addition to thermal stresses, these and other components are also exposed to mechanical stresses and loads that further wear on the components. Such components are exposed to especially high temperatures in first and second stages of turbines.
Many cobalt-based and nickel-based superalloy materials traditionally used to fabricate the majority of turbine components used in the gas turbine engine are insulated from the oxidizing hot gas flow by coating the components with oxidation coatings such as McrAlY or diffusion aluminide, in order to survive long-term operation in this aggressive high-temperature combustion environment.
Thermal barrier coating systems often include three layers, a thermally grown oxide over a metallic bond coat, and a ceramic topcoat over the thermally grown oxide. Typically, the ceramic topcoat is formed from seven weight percent yttria-stabilized zirconia (7 YSZ). The 7YSZ exhibits low thermal conductivity while remaining phase stable at typical operating temperatures seen in gas turbine applications. Ceramic topcoats such as 7YSZ may have limited applicability and can be expensive to apply.
One such metallic bond coat is a MCrAlY coating, where M is iron, cobalt, and/or nickel. Another metallic bond coat is a diffusion aluminide coating, such as NiAl and Ni2Al3. MCrAlY coatings typically exhibit a two-phase microstructure, including β-phase material and γ-phase material. An NiAl beta phase is the aluminum rich phase which provides the aluminum source for thermally grown oxide growth. The presence of γ-phase material increases ductility, thereby improving thermal fatigue resistance. Traditionally, when engines include such MCrAlY coatings along a hot gas path, the coatings can oxidize, for example, when on blades or nozzles exposed to the high temperatures of first stage and second stage temperatures. Such high temperatures deplete β-phase material from the MCrAlY coatings. Upon reaching a predetermined depletion of the β-phase material, such MCrAlY coatings are repaired.
Known MCrAlY coating repair techniques include stripping MCrAlY coatings, for example, with an acid, and re-coating the article with a MCrAlY coating. Such techniques undesirably extend the duration of service periods for turbine components. Such stripping and re-coating can also result in undesirably high costs. Furthermore, improper stripping and re-coating can have an undesirable effect on alloys in the substrate.
Also, aluminide coatings have been limited to certain operational lives at temperatures based upon diffusion thickness limitations and/or may be brittle or produce craze-cracking during service, for example, due to inwardly-formed MCrAlY coatings being over-aluminized.
A MCrAlY-coated article and a process of treating a MCrAlY-coated article not suffering from the above drawbacks would be desirable in the art.